The present invention relates to a semiconductor device and a method for manufacturing a semiconductor device, and more particularly, to a semiconductor device optimal for use in a transfer section of a solid state imaging device, such as a charge-coupled device (CCD) image sensor, and a method for manufacturing such a semiconductor device.
FIG. 1 is a schematic block diagram of a charge-coupled device (CCD) image sensor in the prior art. In FIG. 1, arrows A1 to A3 show the transfer direction of charges. The image sensor includes an imaging section 101 for performing photoelectric conversion with an optical image of an imaging subject, a storage section 102 for temporarily storing charges to be transferred from the imaging section 101, and a horizontal transfer section 103 for transferring charges from the storage section 102 to an output section 104 (output amplifier).
The imaging section 101 and the storage section 102 each include, for example, a three-phase drive vertical transfer CCD (not shown). Terminals P1 to P3 and C1 to C3 are connected to transfer electrodes of the vertical transfer CCDs. The transfer of charges in the imaging section 101 and the storage section 102 is performed in accordance with three-phase drive pulse signals provided to the terminals P1 to P3 and C1 to C3.
The horizontal transfer section 103 includes, for example, a two-phase drive horizontal transfer CCD (not shown). The terminals H1 and H2 are connected to transfer electrodes of the horizontal transfer CCD. The transfer of charges in the horizontal transfer section 103 is performed in accordance with two-phase drive pulse signals provided to the terminals H1 and H2.
The output section 104 receives charges corresponding to an optical image of the imaging subject and converts the charges to a voltage signal in accordance with the number of charges. An external signal processor (not shown) performs an appropriate signal process on the voltage signal.
FIGS. 2A and 2B schematically show the horizontal transfer section 103. FIG. 2A is a plan view schematically showing part of the horizontal transfer section 103, and FIG. 2B is a cross-sectional view taken along line 2B—2B in FIG. 2A. Arrow A3 in FIG. 2A shows the transfer direction of charges in the same manner as in FIG. 1.
As shown in FIG. 2A, the horizontal transfer section 103 includes first electrodes 22 and second electrodes 24 that are arranged alternately. The first electrodes 22 and the second electrodes 24 function as transfer electrodes of the CCD image sensor. An interlayer insulation film 25 (refer to FIG. 2B) is deposited on the first and second electrodes 22 and 24. Contact holes 25a connected to the second electrodes 24 are formed in the interlayer insulation film 25. The second electrodes 24, which are connected to upper layer wires via the contact holes 25a, are electrically connected to the terminal H1 or H2 (refer to FIG. 1) by the upper layer wires.
As shown in FIG. 2B, the horizontal transfer section 103 includes a semiconductor substrate 20, a plurality of insulation layers 21, a plurality of first electrodes 22 arranged on the insulation layers 21, an insulation film 23, and a plurality of second electrodes 24. The first electrodes 22 and the second electrodes 24 are formed from a conductive material such as N-type polycrystalline silicon. The insulation layers 21 and the insulation film 23 are formed from an insulative material such as silicon oxide.
The first electrodes 22 are formed at the same level above the semiconductor substrate 20. Further, the first electrodes 22 are spaced from one another by a predetermined interval. The insulation film 23 covers the first electrodes 22. The second electrodes 24 are arranged along the first electrodes 22. Each of the second electrodes 24 has an upper surface with a recess. Thus, each second electrode 24 has a U-shaped cross-section. Part of each second electrode 24 is located between the adjacent first electrodes 22, and the remaining part of each second electrode 24 is laid above the corresponding first electrode 22 in an overlapping manner. The insulation film 23 electrically insulates the second electrodes 24 from the first electrodes 22.
The interlayer insulation film 25, which is formed from, for example, silicon oxide, is deposited on the first and second electrodes 22 and 24. The contact holes 25a are formed in the interlayer insulation film 25. More specifically, as shown in FIG. 2B, each contact hole 25a is connected to the generally flat bottom of the recess in the corresponding second electrode 24.
Each contact hole 25a is filled with a wiring material, such as tungsten, to form a contact plug. The contact plug electrically connects the corresponding second electrode 24 and upper layer wire.
Japanese Laid-Open Patent Publication No. 2001-308313 describes an example of a semiconductor device in the prior art.
In recent years, to improve image quality, there is a stronger need to miniaturize pixels of a CCD image sensor. However, when pixels are miniaturized, with the structure of FIG. 2, there is a possibility of the connection between the second electrodes 24 and the upper layer wires being unstable. The reason for such instability will now be discussed with reference to FIG. 3.
FIG. 3 is a schematic cross-sectional view showing a miniaturized horizontal transfer section. The miniaturization of pixels reduces the dimensions of the recesses in the second electrodes 24. As a result, it becomes difficult to connect the contact holes 25a to the recesses of the second electrodes 24.
Furthermore, if the processing accuracy is low in a manufacturing process, such as photolithography, a contact hole 25a may be connected to an inclined portion in the upper surface of the corresponding second electrode 24. When such a contact hole 25a is formed by etching the interlayer insulation film 25, there is a risk of the second electrode 24 being over-etched. The broken lines in FIG. 3 show over-etched second electrodes 24.
Additionally, as shown in FIGS. 2B and 3, in the prior art structure, steps St are formed in the interlayer insulation film 25 in correspondence with the shape of lower members. Steps St are also formed above the second electrodes 24. During the lithography process, the steps St remain even after applying a resist to the interlayer insulation film 25. Thus, referring to FIG. 2B, during the formation of the contact holes 25a, the thickness of the resist may be uneven, and light reflection at the interface between the interlayer insulation film 25 and the resist may vary the amount of exposure. This may result in the contacts holes 25a being formed as deficient holes, with, for example, the contact holes 25a having different dimensions.